Didn't know about captain oring. I just made a complete Edgun R5m/Leshiy classic O-ring order from
www.theoringstore.com I was shopping for metric sizes and ive never seen such a complete selection!
Once you know what sizes you need for your gun, either from schematics or parts lists, it's easy to order your own kits. There are usually minimum quantities involved, but prices are cheap!
I just took a look at the captain o ring website. If they made kits for Edguns I certainly would have been tempted to buy one because the engineering has already been done for you except they don't provide an itemized list that tells you which o-ring is for which exact part on your gun. You have to just grab from the bag and match up the ring to the part in hand.
Although they make kits for European airguns such as FX, which would be all metric, they don't appear to list individual metric O-rings for bulk/itemized purchase though. I looked at their bulk o-ring section of the store and all they have are standard ASTM dash# sizes from series -0xx to -3xx. Their kit box service matches an idea that I had to use fishing tackle boxes to make my own custom o-ring kit boxes for my airguns. I would put a label inside the lid of the box showing not only which size o-ring is in which compartment but what component the O-ring fits on for the gun. That would be really cool.
O-rings and more looks like a good source also, if you know what sizes you need and where they go. I think that's the part that trips people up, the time needed to research and find out what size O-rings your particular air gun needs, but you come away with a better understanding when you take the time to figure it out and write it down for your particular airguns.
The minimum shipping charge from the O-ring store is almost $10. O rings and more has a lower minimum shipping rate of $6 and change which may appeal to some depending on the size of your order.